Union officials shared photos of Ryanair employees wearing jeans and sneakers, which is said constituted a substantial violation of safety procedures.

The Green party in the Berlin city government weighed in, telling local daily "Berliner Morgenpost": "If it turns out that the occupational health and safety rules were not respected, this is an absolute no-go."

"In order to prevent further disruption to our customers, we (along with other airlines) positioned a number of fully qualified and authorized ground handling staff to dispatch our Berlin aircraft," Jacobs told DW.

"These union claims are false and their members have no business accessing the airport and ramp when they are not working.

"It's a disgrace that Berlin customers and visitors are having their travel plans disrupted by tiny numbers of unions, who are holding Germany's capital city to ransom and we call on the German government to intervene and prevent any further disruption."

Union representatives said they would not strike for the rest of the week, but warned that the use of strike-breakers meant they may not announce future strikes ahead of time. Previously they announced strikes 18 hours before they came into force.